The present invention relates to a sensor for conductivity and capacitance measurement in gases or liquids.
Combined conductivity and capacitance measurements in fluids are important, for example, in characterizing gasoline/methanol mixtures, ascertaining the water content in brake fluids, analyzing battery acids, and determining oil degradation using a measurement of alkaline reserve. Since conductivity, in particular, exhibits temperature dependency, analyses of this kind should advantageously be coupled with temperature measurements. A combination of conductivity, capacitance, and temperature measurements to characterize gasoline/methanol mixtures is described in the article xe2x80x9cNew Generation of Automotive Sensors to Fulfil the Requirements of Fuel Economy and Emission Controlxe2x80x9d, J. Binder, Sensors and Actuators, Vol. 31, Elsevier Sequola, Lausanne, 1992, pp. 60-67, and has been carried out as a sensor. This sensor is, however, large and very complex in configuration.
The present invention relates to a sensor, in particular for conductivity and capacitance measurement in gases or liquids, having a three-dimensional interdigital electrode arrangement located on a substrate. The sensor additionally has an integrated temperature resistor and preferably also an integrated heating resistor. The temperature resistor, preferably configured in meander fashion, is used to measure temperature in liquids or gases. The heating resistor, also preferably configured in meander fashion, is used to heat the gas and the liquid between the interdigital electrodes or to heat a sensitive material which has been introduced into the electrode structure (for example by pressure technology). Depending on the intended application, for example analysis of gasoline/methanol mixtures, oil monitoring, etc., the heating resistor can be left out in order to simplify the structure.
The present invention advantageously provides for the substrate to be made of silicon, such that the latter can already contain an integrated analysis circuit. Other substrates such as ceramic, glass, or plastics can, however, also be used advantageously depending on the application.
The present invention provides for a miniaturized sensor which can be used to characterize a plurality of liquids and gases, in which the analysis is based on conductivity and capacitance measurements in fluids or gases, either directly or of a chemically sensitive substance between the electrodes. Advantageously, the three-dimensional electrode configuration makes possible a higher sensor sensitivity and lesser sensitivity to interference, as well as miniaturization. The sensor according to the present invention can be manufactured with high precision using a combination of process steps known per se; advantageously, when silicon is used as the substrate, a silicon electronic analysis circuit can also be integrated onto the chip (substrate) in the course of manufacture.
The present invention also concerns an expansion of the sensor depicted, in which a sensitive layer or a multilayer system is provided between the interdigital electrodes. One application that can be implemented, for example, in this context is a moisture sensor, which can be implemented by deposition of a polymer. The water uptake of the polymer causes a change in the dielectric constant, which can be ascertained by way of a change in capacitance. The purpose of the heating resistor located under the interdigital structure is to remove moisture that has collected in the polymer, so as to regenerate the sensitive layer. In addition to the application as a moisture sensor, chemical sensors in the area of fluid or gas analysis are also generally achievable. A series of sensitive layers, for example metal oxides, must be heated for operation thereof, which is controlled by the underlying heating resistor and monitored by the temperature resistor.
The present invention also concerns a method for direct boiling-point determination in liquids. One practical area of application here is, for example, the determination of the quality of brake fluids in motor vehicles. In the measurement method according to the present invention, a small volume of liquid is heated by a microstructured heating resistor, preferably inside a three-dimensional interdigital electrode structure, and the temperature is measured using an integrated microstructured temperature resistor. The interdigital electrode structure is used to determine the capacitance and resistance of the liquid in the presence of direct current and at different measurement frequencies. Since these variables are temperature-dependent, rising temperature results in higher capacitance and lower resistance. Opposite behavior occurs in the vicinity of the liquid""s boiling point. Resistance rises with heating, and capacitance drops. In extreme cases, the liquid boils between the electrodes, which is associated with gas formation. Since the gas bubbles have very different dielectric characteristics from the liquid, unequivocal decreases in capacitance and increases in resistance occur upon boiling. From these changes in capacitance and resistance, the boiling temperature is determined by way of the integrated temperature resistor. An advantage of the sensor, because of the miniaturized structure, is that there is very little beat input into the liquid. In addition, because of the low heat capacity of the overall structure, very rapid measurement (on the order of seconds) is possible.
The present invention also concerns a method for manufacturing a sensor having a three-dimensional interdigital electrode arrangement located on the substrate, in particular an aforementioned sensor, such that a temperature resistor and optionally a heating resistor, preferably made of platinum, nickel, TaNi, or silver, is arranged on one surface of the substrate (the front or back side). A three-dimensional interdigital electrode arrangement, made for example of platinum or gold for highly corrosive media or of silver, copper, nickel, or aluminum for less corrosive media, is arranged on the same or the other side of the substrate. The method according to the present invention provides for a combination of various sputtering, etching, passivating, electroplating, and photolithography steps. The method can also be used, to the extent set forth, for the manufacture of simplified sensors without a heating resistor.
According to a preferred exemplary embodiment, the present invention provides in particular for both surfaces of the substrate to be coated with SiO2 and Si3N4 before application of the temperature resistor onto one surface of the substrate, for example the back side, and of the electrode arrangement onto its other surface.
In particularly advantageous fashion, provision is made for manufacturing the temperature resistor, for example on the back side of the substrate, by sputtering an adhesion layer and a platinum layer onto the SiO2 and Si3N4 layer that is preferably present. Subsequent thereto, a resist material is applied and structured. Etching of the platinum layer to structure the temperature resistor, and passivation of the temperature resistor with SiO2, then take place. Following the passivation step, the present invention provides, advantageously and in a preferred exemplary embodiment, for the temperature resistor to be annealed to establish and stabilize the temperature coefficient.
In a further preferred exemplary embodiment of the present invention, an aforementioned method is provided such that the three-dimensional interdigital electrode arrangement is manufactured on, preferably, the front side of the substrate by first photolithographically defining an electrode region on the front side of the substrate. In the region defined in this fashion, the SiO2 and Si3N4 layers which are preferably present there are etched. An adhesion and electroplating initiator layer is then sputtered on. Following sputtering, a (preferably thick) resist layer for configuration of the electrode arrangement is structured in such a way as to form resist valleys. The resist valleys manufactured in this fashion are plated out with electrode material. The resist mask is then removed and the electroplating initiator layer is etched. After subsequent anisotropic etching of the silicon substrate in the region of the electrode arrangement, preferably the contact connections are exposed and the sensors are isolated.